Conventionally, a protective relay device has been used to collect information such as current or voltage from a power system and detect and cut off failure from the power system in the event of failure in the power system or power facility.
For example, a conventional digital type protective relay device detects occurrence of system fault by performing A/D (Analog to Digital) conversion onto input current signal or voltage signal (analog signal) and using resulting digital waveform data to perform necessary calculation such as protective relay calculation.
Meanwhile, due to progress in information and communication technology in recent years, networks for facilities associated with the power system have been established. For such an exemplary network, a distributed type protective control system has been in practical use. In the distributed type protective control system, functions of the conventional protective relay device are distributed and one or more merging units are typically provided to collect information such as current or voltage from the power system.
A merging unit generates a digital signal through A/D conversion of an input current signal or voltage signal (analog signal), converts the generated digital signal into serial data, and outputs it via a communication line. Based on the serial data, determination as to occurrence of system fault or the like is performed by an intelligent electric device (hereinafter, also referred to as IED), which is a calculation device connected thereto through the communication line.
A protocol of telecommunications standard for serial data transmitted to an IED by a merging unit is defined in IEC61850-9-2. Also, it is defined that a signal employing an operation signal of an IED for control of another device is transmitted in the form of a message type GOOSE (Generic Object Oriented System Event) signal, which is a general object oriented system event.
In other words, it is premised to use the GOOSE signal as a control signal for controlling opening and closing of a circuit breaker provided in a substation in accordance with the output signal of the IED. In order to control devices such as the circuit breaker, the following methods are employed: a method in which a current/voltage signal is received from a merging unit, calculation for protection and control is performed by an IED, and a device is directly controlled by a contact point signal from the IED in accordance with the result thereof; and a method in which the signal is transmitted to the merging unit in the form of GOOSE signal and the device is controlled from the merging unit.
The former is a method that is based on the method of the conventional system. The latter is a method employed to reduce cost of wiring a cable to couple the control signal from the IED to the device by controlling it from the merging unit provided near the device.
A prior art employing the latter method to improve reliability in transmission circuit is disclosed in Japanese Patent Laying-Open No. 2002-315233 (Patent Document 1). This prior art is directed to a system capable of protective interruption even in the event of fault in a process bus or the like by providing two redundant integrated units or merging units, referred to as sensor units, together with transmission paths.